1. Field of the Invention
The present invention relates to a communications system, etc., and more particularly, to a communications system, etc. for making a digital communication by using a frequency-division multiplexing communications method or a code-division multiplexing communications method.
2. Description of the Related Art
In a digital communications system which makes a communication by using a frequency-division multiplexing communications method or a code-division multiplexing communications method, a ratio (PAPR) of transmission peak power to transmission average power becomes large. A transmission power amplifier is designed in consideration of PAPR, leading to a problem that power consumption increases and downsizing becomes difficult.
As a conventional technique for overcoming this problem, a peak reduction technique exists.
Examples of such a conventional technique include the inventions recited in Patent Documents 1 and 2.
According to the invention recited in Patent Document 1, transmission data is OFDM-modulated in an IDFT unit to generate an OFDM signal, whether or not to require a reduction in the generated multi-carrier signal is determined, the multi-carrier signal determined to require the reduction is evenly reduced with an even reduction circuit, and a peak voltage portion is clipped with a clipping circuit, whereby the peak voltage is reduced. In this way, the clipping process is performed if transmission peak power exceeds a threshold value, thereby decreasing PAPR.
According to the invention recited in Patent Document 2, a peak cutting unit reduces a detected peak to a threshold value if the peak of an amplitude level, which is equal to or larger than the threshold value, is detected, and a switching unit outputs the signal, the peak of which is reduced, to an FFT unit.    (Patent Document 1) Japanese Published Unexamined Patent Application No. 2002-77097    (Patent Document 2) Japanese Published Unexamined Patent Application No. 2005-101975
With the conventional transmission power peak reduction technique, the clipping process is performed for a transmission signal. However, since the clipping process is a nonlinear process, inter-symbol interference, etc. can occur, which leads to a degradation of a bit error rate.